1. Field of the Invention
The present invention relates to a semiconductor device including a circuit consisting of thin film transistors (hereinafter referred to as “TFTs”) and a manufacturing method thereof. Further, the present invention, for example, relates to a light emitting device using a light emitting element emitting fluorescence or phosphorescence by applying an electric field to an element having a film including an organic compound (hereinafter referred to as “organic compound layer”) between a pair of electrodes, and to a manufacturing method thereof. Note that a light emitting device in this specification includes an image display device, a luminescent device and a light source (including illuminating devices). Further, a light emitting device includes all of a module of a light emitting device attached with a connector, for example, a FPC (flexible printed circuit) or TAB (tape automated bonding) tape or TCP (tape carrier package), a module provided with a printed wiring board at a front end of the TAB tape or the TCP and a module in which a light emitting element is directly mounted with ICs (integrated circuit) by COG (chip on glass) system.
Note that a semiconductor device in this specification indicates general apparatuses that are capable of functioning by utilizing semiconductor characteristics and includes an electro-optical display device, a light emitting device, a semiconductor circuit and electronic equipment.
2. Description of the Related Art
There is expected application of a light emitting element using an organic compound characterized in thin shape, light weight, high response and direct current low voltage drive as a light emitting body to a flat panel display of next generation. Particularly, a display device arranged with light emitting elements as a matrix shape seems to be superior to a liquid crystal display device of a related art in view of wide viewing angle and excellence in optical recognizing performance.
According to the light emitting mechanism of a light emitting element, it is said that by interposing an organic compound layer between a pair of electrodes and applying voltage, an electron injected from a cathode and a hole injected from an anode are recombined at a center of light emission in the organic compound layer to form molecular exciton and when the molecular exciton returns to the ground state, energy is discharged to emit light. There are known singlet excitation and triplet excitation in excited state and it seems that light can be emitted by way of either of the excited states.
It is possible to use driving methods of passive matrix drive (simple matrix type) and active matrix drive (active matrix type) for such a light emitting device formed by arranging light emitting elements in a matrix shape. However, when a pixel density is increased, the active matrix type light emitting device provided with switches for each pixel (or dot) is advantageous since the active matrix type light emitting device can be driven at low voltage.
Further, although a low molecular system and high molecular system (polymer system) material have been respectively researched for the organic compound for constituting the organic compound layer (strictly speaking, light emitting layer) regarded to be the core of a light emitting element, more attention is given to the high molecular system material facilitated to handle and having high heat resistance than the low molecular system material.
Further, although there are known methods such as vapor deposition method, spin coating method and ink jet method for a film formation method of the organic compounds, as a method for realizing full color formation by using the high molecular system material, the spin coating method and the ink jet method are particularly well known.
The light emitting elements having the organic compound has a defect that is easy to be deteriorated by various factors, therefore it is a maximum object of the light emitting elements to be formed with high reliability (long lifetime).
A light emitting element having an organic compound is easily deteriorated mainly due to water and oxygen, and as a failure state caused by these causing factors, a state in which the lowering of brightness has partially occurred or a non-light emitting region has been generated is seen.
Moreover, a state in which the expansion of the non-light emitting region progresses is also seen as the changes occurring only along with the passing time, or as the changes occurring when the time passes while the light emitting element is driven. Particularly, in the case where anon-light emitting region is generated at the stage immediately after a light emitting element having an organic compound has been fabricated, the expansion of the non-light emitting region often progresses along with the time passing, and it may be also seen that it progresses further until finally the entire region deteriorates to be the non-light emitting region.
Moreover, the non-light emitting region is easier generated from the circumferential portion of the light emitting region, and since the expansion of the non-light emitting region progresses along with the time passing as if the light emitting region shrinks, this failure mode is referred to as a shrink. It should be noted that in FIG. 1A, a light emitting state immediately after the light emitting element has been fabricated is shown, and that in FIG. 1B, how the shrink was generated when the time further passed immediately after the light emitting element had been fabricated is shown. FIG. 11B shows an example in which the expansion of the non-light emitting region is uniformly generated after the non-light emitting region has been generated from the circumferential portion, however, it may occur that the circumferential portion of the light emitting region is in convex and concave shapes by the ununiformly progressing shrink.
Because the light emitting area reduces, particularly in such a case where the area of the light emitting region is as small as that of an active matrix type light emitting device, these failures mean that a certain light emitting element becomes a non-light emitting element at an early timing. And further, in the case where the area of the light emitting region is small, if the light emitting area is reduced, the ratio occupied by the non-light emitting region is enlarged. Therefore, in the case where a display device is fabricated using a light emitting element, it is difficult to obtain a highly-defined (whose pixel pitch is small) and highly reliable display.
Moreover, a non-light emitting region just like the sunspot may be generated immediately after a light emitting element having an organic compound has been fabricated, this failure mode is referred to as a dark spot. Moreover, this dark spot may be also expanded along with the time passing. It should be noted that FIG. 12A is a figure showing that a dark spot exists at the third one from the top of the left row, at the first one and sixth one from the top of the right row immediately after the light emitting element has been fabricated, and how the expansion of the dark spot is generated when the time passed from the time immediately after the light emitting element has been fabricated is shown in FIG. 12B. In FIG. 12B, a shrink is also generated at the same time with the, expansion of the dark spot.
An object of the present invention is to reduce or eliminate the occurrence of the above-described various failure modes in a light emitting element having an organic compound.